1. Field of the Invention
The present invention relates generally to radiation amplitude control or stabilization systems for antenna elements, and particularly to a radio frequency (RF) amplitude stabilization system for antenna elements of an active phased array antenna.
2. Description of the Known Art
In an active phased element antenna array, a number of antenna elements are each driven by an associated RF amplifier that may be in the form of a GaAs integrated circuit. The phase of RF energy applied to the input of each amplifier is controlled to provide a desired beam direction for the sum of the RF energy radiated by all elements of antenna array at a given moment. The amplitude of RF energy fed to each individual element is often pre-set to obtain a desired "taper", for example, with elements at the center portion of the array excited at a higher level of RF energy than elements situated at the outer periphery of the array.
In the past, conventional RF or microwave power dividers were used to implement a desired array taper. For active arrays, however, the amplitude taper is controlled, to a large degree, by the tolerances of the array components themselves, which are not sufficient to realize the accuracies required in applications such as microwave landing systems (MLS) at reasonable cost. Such systems require at least two phased array antennas adjacent an aircraft runway for causing both an azimuth (AZ) beam and an elevation (EL) beam to be scanned rapidly "to" and "fro", and "up" and "down", respectively. Equipment on board an aircraft approaching the runway receives the beams as scanned by the antennas and, based on synchronization information obtained from a third runway antenna, calculates the aircraft heading and angle of descent relative to the runway.
Understandably, malfunctions in the array elements that might cause an error in the beam steering operation of either the AZ or the EL antenna, will result in the computation of false positional information by the aircraft equipment during the critical runway approach time of flight.
A known method of securing a pre-set amplitude taper for the elements of a phased array antenna involves implementing a leveling or AGC loop to control the output power of the amplifier associated with each element. This technique has, however, two limitations, namely (1) loop response time, and (2) complexity. A third problem involves the accuracy with which the desired radiated RF energy from each element can be set. MLS applications require that such power setting accuracy be on the order of 1.5 db.
Another element tapering method involves "tweaking" each element to give the proper output power. This method is expensive and has limited dynamic and temperature ranges.
It is, therefore, an object of the present invention to overcome the above and other shortcomings of known methods of establishing a desired amplitude taper for antenna elements in a phased array.
A further object is to provide a technique that allows setting a desired RF output power level from an antenna element with high accuracy.
Another object of the invention is to provide a system in which the amplitude of RF power radiated by an antenna element is precisely controlled through digital techniques.